Podcaster: Richard Drumm
Q&A: What are transition disks?

Organization:365 Days Of Astronomy

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Description: This is a story about exoplanet formation. We all have to start from something, and planets are no exception

Bio: Richard Drumm is President of the Charlottesville Astronomical Society and President of 3D – Drumm Digital Design, a video production company with clients such as Kodak, Xerox and GlaxoSmithKline Pharmaceuticals. He was an observer with the UVa Parallax Program at McCormick Observatory in 1981 & 1982. He has found that his greatest passion in life is public outreach astronomy and he pursues it at every opportunity.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by — no one. We still need sponsors for many days in 2016, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at

This is the 365 Days of Astronomy Podcast.

In Q&A today we answer the question “What are transition disks?”

This is a story about exoplanet formation. We all have to start from something, and planets are no exception.

Our solar system formed from a rotating cloud of gas & cosmic dust nearly 5 billion years ago. The center part of the cloud collapsed under its own gravity to form good old Sol, our Sun.

The rest of the cloud formed a flattened disk of material orbiting the Sun like the way Saturn’s rings orbit Saturn.

None of us was around then, so we don’t have any pictures of the formation of our home planet, but we do have a few similar systems in the galaxy that we can study.

In the southern constellation of The Chameleon is a star called T Chamaeleontis, or T Cha for short. It’s a T Tauri type variable star, has 1.5 times our Sun’s mass, is 350 light years away and is just a baby, at 7 to 10. . . million years old.

The star has an inner ring, with a large gap separating it from an outer ring. This type of disk is called a Transition Disk.

The star’s circumstellar disk is in transition from a solid disk to a stellar planetary system and large gaps have begun to form in the formerly solid disk.

So it’s like a toddler of sorts, learning how to walk. It’s making the transition to a fully fledged planetary system.

Transitional disks don’t radiate strongly in the mid-infrared wavelengths. This is one of the hallmarks of the type. It’s a sign of the creation of gaps in the disk.

The gaps in the disk could possibly be caused by one of the following processes:
1. The grains of cosmic dust have clumped together and formed larger grains, pebbles, boulders and even protoplanets.
2. A large planet orbiting the star has gravitationally cleared up the lane, either by residing in the lane or by perturbing the dust there with an orbital resonance.
3. The star’s equivalent of solar wind has blasted the area clear, sublimating any ice crystals there and driving the dust farther out by radiation pressure.

But which one is the cause? Or is it all 3?

A team of astronomers from the NOAO, the Harvard-Smithsonian Center for Astrophysics and the Space Telescope Science Institute set out in 2014 and 2015 to find out.

They didn’t study T Cha, but studied similar stars in Taurus & Ophiuchus. They found a relation between the mass of material that’s being accreted onto the parent star and the mass of the dust disk.

This relation suggests that the reason for the disk’s gap is option #2, the formation of one or more gas giant planets in the system.

Then a German/Italian/French and Chilean team in 2016 & 2017 studied T Cha’s disk. They determined that the gas giant can be no larger than 8.5 times Jupiter’s mass and is likely closer to 2 times Jupiter’s mass.

There is a great deal of interest in the planetary formation mechanisms that the various protoplanetary disks exhibit.

The most dramatic example of a protoplanetary disk we’ve recently seen is the pre-transitional Class I disk of HL Tauri.

Google HL Tau and you’ll see the image that ALMA shocked the astronomical world with back in 2014. At only 1 million years old previous models said it couldn’t be forming planets yet.

But clearly it has started forming some fine examples of planets. We haven’t imaged the planets yet, but we can see the cleared-out lanes in the dust as plain as can be!

We have imaged a couple planets in dust disks, though. Beta Pictoris and Fomalhaut have whetted our appetites for more! Stay, uh, “tuned” here for more exciting news in the future!

So it seems astronomers have a great deal of work to do before the formation of planets is completely understood. Perhaps one of you listeners will take up the mantle and figure all this out!

Thank you for listening to the 365 Days of Astronomy Podcast.
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365 Days of Astronomy
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